Method and Apparatus for Determining Intra Ocular Pressure of an Eye

ABSTRACT

A tonometer, which uses an applanation prism that reflects light inversely related to an applanated area of the cornea, determines intraocular pressure (IOP) by making initial contact with the cornea, and then pressing the prism against the eye through a predetermined departure range from the initial contact value. The departure can add a predetermined prism pressing force beyond the initial contact value or can increase prism pressing force sufficiently to produce a predetermined increase in the applanated area. From the change in applanated area and prism pressing force required, the microprocessor can be programmed to determine IOP in several ways that do not suffer inaccuracy from variations in tear volume and corneal thickness and curvature in different eyes being examined.

REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of co-pending application Ser. No. 10/178,987, filed 25 Jun. 2002, entitled “Method and Apparatus for Examining an Eye”. The aforementioned application is hereby incorporated herein by reference.

TECHNICAL FIELD

Eye examining instruments and methods.

BACKGROUND

Our previous U.S. Pat. Nos. 5,070,875, entitled “Applanation Tonometer Using Light Reflection To Determine Applanation Area Size”, 6,179,779, entitled “Replaceable Prism System For Applanation Tonometer”, 6,471,647, entitled “Method of Operating Tonometer”, and 6,736,778, entitled “Replaceable Prism For Applanation Tonometer” suggest tonometers, tonometer operating methods, and tonometer prisms for measuring intra ocular pressure (IOP) of an eye. Our type of applanation tonometer has an actuator that presses a prism with a variable and determinable force against a cornea of an eye being examined while a source directs light to reflect from an applanation surface of the prism to a detector producing a detected light signal received by a microprocessor.

Our experiments and experiences with working prototypes improving upon the disclosures of our issued patents and pending applications have led to several related discoveries. We have found that by changing and adding to the eye examining procedures instruments such as ours have made possible, we can calculate IOP more accurately. These changes involve not only improved eye examining methods, but also programming or otherwise structuring a tonometer to perform improved methods leading to information that has not previously been clinically available.

SUMMARY

The tonometers that are commonly used clinically have operated only during a diastolic phase and have measured only a diastolic intra ocular pressure (IOP). While our instrument accomplishes a diastolic IOP measurement, we have discovered that our instrument can also produce a useable signal during a systolic pulse occurring in an eye being examined. Upon exploring this, we found that a systolic phase signal from our instrument can be used to determine an ocular pulse pressure or a systolic IOP. This constitutes valuable additional information not obtainable with previous clinical tonometers. It provides a measure not only of diastolic IOP, but also of systolic IOP, and enables an average, weighted average, or mean IOP determination that more accurately represents the true or complete IOP experienced by the eye being examined.

Other experiments with IOP signals attainable from our instrument have led to eye examining methods differing from our previous suggestions. We have found, for example, that t a prism pressing force variation range for IOP examining purposes can begin with an initial contact value and can change from that value through a predetermined range of change, rather than proceeding from a reference applanation area to a measurement applanation area. This method eliminates from the usable signal the variations in biomechanical properties of a specific cornea of an eye being examined, since such biomechanical variations are automatically involved in the initial contact value, which is subtracted or excluded from the usable signal. These corneal biomechanical properties, which vary from one eye to another include tear volume that supplies a surface tension force drawing a prism applanation surface into contact with the cornea and corneal properties that affect its elasticity or deformability, including corneal thickness and corneal curvature. All such variations affect the initial contact value and are thereby subtracted from an IOP determination that does not include or rely upon the initial contact value.

Experience with systolic pulse signals produced by our instrument has led to discovery of other measurements available from examining an eye. We found that we can determine ocular blood flow derived from the departure of the detected light signal from the diastolic IOP during the systolic pulses. Moreover, we have found that we can determine a tonography measure from the way the detected light signal changes from a systolic pulse back to the diastolic phase. We can also determine tonography by measuring a preceding IOP; then pressing the prism against the eye with a predetermined force sufficient to raise the IOP for a predetermined interval; followed closely by determining a subsequent IOP. By doing this we can derive the tonography measure from the differences between the preceding and the subsequent IOP determinations. An ocular blood flow measurement and a tonographic measurement of the effectiveness of an eye's trabecular meshwork add significant and previously inaccessible diagnostic information of value to a clinician.

Finally, to ensure that the additional information produced by our eye examining method and instrument is readily available to clinicians, we have made our instrument fast acting, compact, convenient, and objective in its operation. Besides producing much new information, our instrument automatically rejects false readings, and automatically requires concentric contact with a cornea at a proper orientation to attain an accurate reading. The programmed microprocessor in our instrument can preferably store, send, and receive information to perform all the required tasks and operations and to co-operate with computers and other information processing equipment.

DRAWINGS

FIG. 1 is a schematic view of a preferred embodiment of our improved tonometer, which is suitable for practicing our inventive eye examinations.

FIG. 2 is a schematic diagram of a portion of the tonometer of FIG. 1 involving a prism applanating an eye, a light source, a detector, a microprocessor, and an output.

FIGS. 3-8 are schematic graphs of detector signals produced pursuant to our inventive eye examinations.

DETAILED DESCRIPTION

Our eye examining method requires an instrument that can produce a signal representing intra ocular pressure (IOP) of an eye being examined. Such instruments are normally called tonometers, but our instrument and the ways it can be used produces information going beyond what can be expected of previous tonometers. Several variations of tonometers suitable for our purposes are described in our previous patents and applications. A presently preferred embodiment of such a tonometer 10 is schematically represented in FIGS. 1 and 2. Tonometer 10 preferably includes an actuator 15 pressing a prism 30 with a variable and determinable force against a cornea of an eye 40 while a light source 20 directs light to reflect from an applanation surface 31 of prism 30 to a detector 25. In response to received light, detector 25 produces a detected light signal that is electrical in form and is sent to and analyzed by microprocessor 50, which in turn controls actuator 15 for varying the force of prism 30 pressing against the eye 40 and for varying the energization of light source 20. Microprocessor 50 also preferably drives a display 51 and preferably communicates with output and input devices 52.

The essential components of tonometer 10 include microprocessor 50, prism 30, some form of actuator 15, a light or radiation source 20, and a transducer or detector 25 receiving light or radiation reflected from applanation surface 31 and sending a corresponding electric signal to microprocessor 50. The precise working relationships among these essential components can be varied considerably, however, and the schematic illustrations of FIGS. 1 and 2 show only a presently preferred embodiment.

Prism 30 is preferably replaceable and disposable so that it can be removed from prism holder 32 after each examination and replaced with a fresh prism. This ensures that infectious agents, including the possibility of prions, are not transmitted from one pair of eyes to another.

Prism holder 32 is preferably mounted on arm 12, which is arranged to rotate or turn slightly around pivot 13. Since only a few millimeters of movement back and forth of prism 30 is required, as indicated by the double headed arrow, the rotational turning of arm 12 is slight.

A counter balance 14 arranged on arm 11 is disposed at a suitable moment arm from pivot 13 to return prism holder arm 12 to a base position. Proper arrangement and balancing of arms 11 and 12 around pivot 13 can eliminate the need for any return spring, and can enable instrument 10 to operate in different orientations. The accurate counterbalancing of prism 30, as accomplished by element 14, allows tonometer 10 to be brought into a position in which prism 30 engages an eye without applying any pressure to the eye.

Actuator 15 can be any of a variety of motors and other preferably electromagnetic prime movers. The preferred actuator schematically illustrated in FIG. 1 includes a coil 16 that is moveable relative to a permanent magnet 17, depending on the power supplied to coil 16. The relatively lighter coil 16 is preferably fixed to arm 12, and the relatively heavier permanent magnet 17 is preferably fixed to a body of instrument 10, because this reduces the rotating mass. It might also be possible to arrange permanent magnet 17 or coil 16 in the position of counter balance 14 to further reduce the rotational mass. Many other possibilities exist but for sake of conciseness and simplicity these have not been illustrated.

Internal wiring within instrument 10 preferably connects a power supply (not shown) with microprocessor 50, which powers actuator 15, light source 20, and light detector 25. Microprocessor 50 preferably drives display 51 to display information directly to an instrument operator, and microprocessor 50 preferably has connections enabling it to receive and output information to and from other devices such as computers, keyboards, and number pads 52. Microprocessor 50 is preferably programmed to accomplish all the preferred operations of tonometer 10 and to calculate IOP on the signal information resulting from an eye examination.

The light emitted by source 20 is preferably in a visible red region of the electromagnetic spectrum, but other colors of visible light are also possible, both coherent and incoherent, as is radiation energy at infra red or microwave frequencies. For simplicity, all of these possible different radiation frequencies are characterized as “light” within this application. As is well known, use of electromagnetic radiation at different frequencies can require angular adjustments so that light within prism 30 that is internally incident on applanation surface 31 is internally reflected to detector 25 except for a portion of the light that transmits into the wetted area of an applanation region of contact with prism surface 31. Of course, any radiation entering eye 40 must not cause injury.

Prism 30 is preferably molded of resin material to have a low enough manufacturing cost to be affordably disposable. Much more information on preferred characteristics of prism 30 is available in our U.S. Pat. No. 6,179,779 and No. 6,736,778. As we have previously suggested, microprocessor 50 is preferably programmed to require that prism 30 be replaced after being used for examining a pair of eyes. This is intended to prevent the tonometer prism from transporting infectious agents from the eyes of one person to the eyes of another.

Experiments and clinical experience with a working prototype of a tonometer instrument such as shown in FIGS. 1 and 2 has led to an improved way of measuring an intra ocular pressure (IOP). Our previous patents and applications recommended determining IOP from prism pressing force required to change between a reference applanation area (with a corresponding reference signal) and a measurement applanation area (with a corresponding measurement signal). We have now found that it is not necessary to use predetermined reference and measurement areas and corresponding signals to determine IOP.

Our new method, which we program into microprocessor 50, preferably begins with a pre-contact value 23 of a detected light signal recognized by microprocessor 50 before prism 30 contacts eye 40. This signal is illustrated in FIG. 8. When the prism initially contacts its applanation surface with the cornea of an eye, the pre-contact signal decreases to an initial contact signal 27, also shown in FIG. 8. This occurs because initial contact wets the applanation surface 31 and slightly applanates the cornea.

The initial contact signal preferably occurs without prism 30 being pressed against a cornea by actuator 15. The only contact pressure between prism 30 and eye 40 arises from surface tension of tears in the eye that draw the applanation surface against the cornea. The counterbalancing of prism 30, as shown in FIG. 1 and as explained above, allows prism 30 to move slightly back and forth so that as instrument 10 is brought into proximity with an eye of a patient, prism 30 is free to move somewhat upon initial contact with the cornea of an eye, without applying any prism pressing force. In this circumstance, surface tension of the tears in the eye being examined pulls the prism against the cornea with a very slight force resulting in the initial contact value.

The tonometer 10 can be programmed to recognize that the preliminary or initial contact value 27 has occurred by noting the sudden reduction in the detected signal that occurs when the prism contacts the eye. Before this happens, all the light incident on prism applanation surface 31 is internally reflected as signal 23, but when surface 31 contacts a cornea, a portion of the prism applanation face is wetted by tears on the cornea so that light within the prism escapes into the wetted area leaving only a noticeably diminished light signal 27 reflected to the light detector 25. This reduced preliminary or initial contact value signal 27 can vary from eye to eye, as explained below, but also serves advantageously as a starting point for an IOP determination.

Variations in the initial contact value 27 occur from differences in the tear volume of different eyes being examined, and differences in the biomechanical properties of the corneas being contacted. These biomechanical properties include corneal curvature, thickness and elasticity. A thin or somewhat flattened cornea is softer and more easily deformed and can produce a larger reduction in the detected signal upon initial contact with the prism face. Tear volume variation changes the amount of surface tension pulling the prism against the cornea, which also affects the variations due to corneal curvature, thickness, and elasticity. Conversely, a thicker or more arched corneal surface is more resistant to deformation and will produce a smaller reduction in detected signal when contacted by the prism face. These differences from one eye to another are a potential source of inaccuracy in measuring IOP, but our measuring method subtracts or eliminates the effect of these variations to produce a more accurate determination of IOP.

We take advantage of the initial contact signal 27, with its inherent inaccuracies, by producing measurement signals that extend through a predetermined departure range from the initial contact value 27 as a measurement base, and we preferably use the same predetermined departure range for each eye being examined. The departure range can be defined by a predetermined increase in the pressure applied in pressing the prism against the eye, or can be defined by a predetermined increase in the applanated area, causing a predetermined reduction in the detected signal. Each of these departure ranges is represented by signal steps 26 reducing or leading downward from initial contact value 27. Steps 26 preferably occur quickly, such as at 13 millisecond intervals, and the first one or two steps can be subtracted or ignored in the IOP determination that microprocessor 50 accomplishes.

The measurement range thus has a fixed or predetermined dimension in terms of applied force to press the prism against the cornea to change the applanation area, but each of these is used as a departure from the initial contact value 27 and neither result in any predetermined values at ends of the departure range. Since the initial contact value 27 is variable and thus affords a variable starting point for each measuring range, the opposite end of the measuring range is not predetermined, but the length of the range in terms of prism pressing force or change in applanated area size is the only predetermined factor. Any value at the end of the range extending from the initial contact value is not a fixed value, but is based only on distance from the initial contact value. The stepped signal 26 that occurs over the pre-determined departure range is also free of the inevitable errors that would otherwise occur in using initial contact signal 27. Such errors, based on tear volume and the biomechanical properties of the cornea of eye 40 are automatically subtracted by determining IOP only from signals generated within the predetermined departure range.

To accomplish this, we program microprocessor 50 to operate actuator 15 to press prism applanation surface 31 against the cornea with increasing force applied during a time interval to change the detected light signal through the predetermined range of values extending from the preliminary value. In doing this, microprocessor 50 preferably energizes actuator 15 to apply increasing prism force in predetermined increments that are applied in predetermined brief time intervals so that the detected light signal 26 varies in a step wise, sloped configuration as graphically illustrated in FIGS. 3 and 8. This has the advantage of making a total number of applied prism force increments equal to a total number of time increments. Time increments or steps can then be counted to determine prism force change.

Prism force changes and time intervals can also be continuous or analog, rather than being broken into the preferred increments. Prism pressing force producing the excursion of signal 26 over the preferably stepped range can also be made with increasing signals from pressure reductions as well as decreasing signals from pressure increases, and the signal collecting process can be continued long enough to encompass at least two systolic pulses 28 that are illustrated separately in FIG. 8. Our present preference is to use several excursions back and forth between the variable ends of the predetermined departure range to collect corresponding signal changes that include systolic pulses 28.

Prism force changes with respect to time are preferably linear, rather than nonlinear, to simplify slope determinations. An exception to this occurs if the instrument is measuring an exceptionally hard eye and the departure range is based on a predetermined increase in applanated area during prism pressing force increases. If the usual incremental pressure force applied to the prism is not producing much change in the applanation area, instrument 10 can be programmed to increase the force of the steps to achieve the predetermined amount of applanation area change. This can make the prism pressing force changes non-linear.

Signal 26 and the data it represents then provides several ways of determining IOP. One way is to proceed with increasing the prism pressing force for a predetermined number of increments or for a predetermined time interval resulting in a predetermined increase in prism pressing force above the initial contact value. Our program for microprocessor 50 then determines IOP from the signal change reached at the end value of the predetermined range of prism force change values. By a similar method, the force change values can be continued until the detected light signal 26 (and corresponding applanation area) reaches a predetermined departure from the initial contact value, with microprocessor 50 determining IOP from the total pressure force required to reach the end signal value. Such an end signal value differs from the previously suggested measurement signal value by being a predetermined departure from the variable initial contact signal value 27 rather than being an absolute or independent value.

Another way that microprocessor 50 can determine IOP from detected light signal 26 is by measuring or detecting the slope as signal 26 changes over time or force intervals within the predetermined departure range. In effect this involves a ratio of actuator prism pressing force to applanation area increases during variation within the departure range from the initial contact value.

We prefer that each measurement include several traversals of the prism pressing force range or the applanation area change range, and these traversals can be both in the direction of increasing force and decreasing force. Traversals of the predetermined departure range can thus move toward decreasing and increasing signal strength for several passes to generate a statistically significant amount of data in a few seconds. This can allow discard of any data points that are inconsistent, such as might occur from an eye blink or other movement of the eye or the instrument, and the data can otherwise allow averaging. Since systolic pulses will occur during the measurement, microprocessor 50 can be programmed to calculate a mean IOP that takes into account the increased intraocular pressure during systolic pulses and the rate at which such pulses occur.

From an ophthalmologically known relation between IOP and force used in applanating a corneal area we have been able to apply linear, logarithmic, and logistic regression analyses to calculate IOP from the signals produced by instrument 10. Especially logistic regression analysis, which allows us to consider several variables at a time, has been useful in making force-to-signal-to-IOP calculations. We have also corroborated these results by manometric comparisons and Goldman tonometer readings.

Microprocessor 50 is preferably not programmed to make sophisticated mathematical calculations itself. We prefer instead that signal analysis be done separate from microprocessor 50, which is then programmed or loaded with a look up table from which it can determine IOP based on signal values. In doing this, microprocessor 50 can be programmed to average an IOP determination made by different methods.

These IOP determining methods can also be combined or used in conjunction so that one determination corroborates another. Moreover, elapsed time required to reach an end value can serve as another corroborator of an IOP determination. All the IOP determining methods can be combined in a single instrument that determines IOP according to each method and corroborates by comparing elapsed time, force change and electrical signal change. Any differences in the IOP determinations can be averaged out, unless differences are unusually large, in which case the microprocessor can be programmed to repeat the measurement. In a similar way, if elapsed time casts doubt on the accuracy of an IOP determination, the examination can be repeated.

These methods of IOP determination, besides being simple, accurate and fast, have another important advantage. By monitoring signal change relative to an initial contact value 27 that is not fixed but is related to each eye being examined, these IOP determining methods automatically subtract the effect of variations in corneal curvature, thickness and tear volume. Our previous IOP determining suggestions envisioned separate measurements of corneal curvature and corneal thickness and input of such measurements to adjust IOP determinations. This is no longer necessary with our present IOP determining methods, because the initial contact signal value 27 is not a fixed value but is allowed to vary with each eye being examined. In effect, the initial contact value, which is necessarily variable depending on the tear volume and the different biomechanics of the corneas of the eyes being examined, is subtracted from our calculations so that an IOP determination is not affected by any of the variables involved in the initial contact value.

This variation is schematically illustrated in FIG. 3 by the difference between initial contact value signals 27 and 27 a, and the corresponding difference between detected signals 26 and 26 a. More specifically, initial contact signal value 27 can be seen as a typical signal produced by initial contact of the tonometer prism with the cornea of a normal eye having a normally curved or arched cornea and a normal cornea thickness. Initial contact value signal 27 a then indicates an initial contact signal from a cornea that is flatter or less arched than normal or a cornea that is thinner than normal. The value of initial contact signal 27 a being less than the value of initial contact signal 27 indicates that initial prism contact with the thinner or flatter cornea has applanated a larger area. It would also be possible, but is omitted for the sake of simplicity, to show another initial contact value signal larger than signal 27, indicating initial contact with a cornea that is more arched than normal or is thicker than normal, resulting in applanation of a smaller area.

The ophthalmological literature indicates that every 50 micrometer variation in corneal thickness produces a 1.5 mm Hg variation in measured IOP. This variation will automatically appear in initial contact value signal 27 to adjust the starting point for a predetermined departure range of values. Similar indications are available in the ophthalmological literature for the effects of differences in arching or curvature of the cornea. These two translate into differences in measured IOP, which are automatically compensated for by initial contact signal value 27.

The variations in initial contact signal 27 due to corneal characteristics, such as curvature and thickness, do not matter in our present method of determining IOP, because change in detected light signal 26 or 26 a, proceeds through a predetermined range departing from whatever initial contact value occurs. This automatically subtracts or eliminates corneal curvature and thickness as possible variables in an IOP determination. The result is a more accurate IOP determination that does not have to be adjusted for corneal thickness or curvature and does not require any separate measurements of or adjustments for corneal thickness and curvature. This advantage can be especially important in examining eyes whose corneas have been modified for vision correcting purposes. The fact that such corneas can respond as thinner than normal does not prevent our tonometer from accurately measuring IOP.

We have also found that our tonometer instrument detects systolic pulses during eye examinations. Goldmann tonometers, which are ubiquitous in opthalmological clinics, cannot make an IOP reading during a systolic pulse. Our instrument, in contrast, produces a detected light signal during both a diastolic phase and a systolic phase. This has led to several new ways of determining IOP, one of which is schematically graphed in FIG. 4.

The process illustrated in FIG. 4 occurs after making a determination of diastolic IOP. For doing this, microprocessor 50 is preferably programmable to allow an operator to secure a reading only of diastolic IOP. Ocular pulse signals occurring during a systolic phase are then ignored by microprocessor 50 in making an IOP determination. Microprocessor 50 can also be programmed to proceed in several ways to determine a diastolic IOP while also taking into account the detected signal departure from a diastolic phase 26 during a systolic pulse.

For the method illustrated in FIG. 4, microprocessor 50 is programmed to operate actuator 15 to hold prism 30 against a cornea with a predetermined force for a long enough duration for two systolic pulses 28 to occur. These are illustrated in FIG. 4 as saw-tooth shaped increases in the otherwise flat diastolic signal 26. The height of systolic pulse signals 28 above diastolic signal 26 indicates ocular pulse pressure (OPP), and this is preferably used as an ingredient in an IOP determination. The fact that systolic pulse signals 28 are larger or higher than the diastolic phase signal 26 is because a systolic pulse slightly hardens the eye being examined, which reduces the area applanated by the tonometer prism 30 and increases the detected light signal.

The eye is subject not only to the IOP occurring in a diastolic phase, but also to the increased IOP that occurs during a systolic pulse when a bolus of blood flows into the eye. An IOP determination based only on a diastolic phase measurement therefore does not indicate the true pressure experienced by the eye over time. For example, a diastolic pressure might be 20 mm of mercury, while systolic pulse pressures reach 26 mm of mercury. A true IOP reading for all the pressure experienced by the eye over time should then include the systolic pulse pressures as a factor increasing the IOP over the diastolic phase pressure.

Broken line 29, as illustrated in FIG. 4, indicates an average, weighted average, or mean IOP determined not only from diastolic phase signal 26, but the increased height of ocular pulse signals 28, and a pulse rate. Microprocessor 50 is preferably capable of measuring time intervals and of calculating a pulse rate from the time elapsed between a pair of OPP signals 28. Microprocessor 50 then preferably determines a mean or average IOP having a value higher than a diastolic phase measurement, based on the height and frequency of systolic pulse pressures. The systolic or OPP is calculated in a manner similar to the calculation of the diastolic IOP. The OPP can also be calculated as a percentage of the deviation from the diastolic phase IOP. Various regression analyses can be applied to these calculations, which can be corroborated manometrically.

Microprocessor 50 can also be programmed to determine only a systolic IOP, which can be done by ignoring diastolic signal phase 26. For most purposes, though, an average, weighted average, or mean IOP determination based on both diastolic and systolic phase signals is preferred.

The ability of our instrument 10 to produce usable systolic pulse signals 28 can also be used to determine ocular blood flow (OBF). This is preferably determined from the height of systolic pulse signals 28 above diastolic base line 26. The ophthalmological literature includes suggestions for calculating ocular blood flow from ocular pulse pressure by using a Friedenwald equation or one of the suggested modifications of the Friedenwald equation. Most of these suggestions focus on the height of signal 28 above a diastolic base line 26, as we prefer. Some suggestions have also focused on a leading edge of signal 28, and at least one proposal, which we have not adopted, suggests that the area under the signal 28 above the diastolic baseline 26 be considered. The various forms of regression analysis can also be applied to ocular blood flow calculations, and these can be corroborated by clinical experience. Whatever calculations are used, the results are preferably translated into a look up table programmed into microprocessor 50 for use in outputting OBF information.

Another measurement that becomes possible from the availability of OPP signals 28 is a tonography measure of the effectiveness of the trabecular meshwork of the eye being examined. We prefer determining this from the downward or trailing slope 24 of the OPP signal 28 as it returns from a peak pressure back to the diastolic base line 26. Generally, a steeper downward slope 24 indicates an effective trabecular meshwork that quickly returns from an elevated OPP back to a diastolic level. Conversely, a more gradual and extended downward slope 24 indicates a trabecular meshwork that is more impaired and recovers more slowly from a systolic pulse pressure.

Methods of calculating tonography measures from applanation signals are also available in the ophthalmological literature. These generally agree that down slope 24 is a key ingredient for tonography calculations. Like the IOP, OPP, and OBF calculations, tonography measures can be refined by regression analyses and can be corroborated by clinical experience and by other measures; they are preferably translated into a look up table programmed into microprocessor 50.

The availability of a tonography measure, along with an OPP measure and an OBF determination gives a clinician considerably more information than has been available from tonometers. This additional information is valuable for both diagnostic and treatment purposes. Knowing the value of systolic pulse pressures and average or mean IOP gives information that is helpful in setting the aggressiveness of treatments used to reduce IOP. It can also help to determine whether to treat with drugs aimed at improving the effectiveness of the trabecular meshwork or whether to treat with drugs aimed at slowing down the production of ocular fluid. For example, using tonometer 10 to produce both an IOP determination and a tonography determination can affect a treatment method for an eye being examined. If the IOP determination is high enough to warrant treatment and the tonography determination is normal, then a clinician would treat the eye with a drug aimed at reducing production of aqueous fluid. On the other hand, if the IOP determination is high enough to warrant treatment and the tonography determination is subnormal, meaning that the trabecular meshwork of the eye is performing at a rate less than normal in removing aqueous fluid from the eye, then a clinician would treat the eye with a drug aimed at improving performance of the trabecular meshwork.

Knowing a measure of OBF can be relevant to these choices because some drugs can reduce ocular blood flow as a side effect. This must be guarded against so that OBF is not reduced enough to impair the health of the optic nerve and other eye components. Some drugs used in glaucoma treatment are either known or suspected of reducing OBF as a side affect, and having an OBF determination readily available can be used to avoid such drugs in a treatment aimed at reducing IOP.

Some drugs are also known to improve blood circulation generally, and these can be used if an OBF determination indicates that ocular blood flow of an eye being examined could advantageously be increased. This is the case with normal pressure glaucoma that impairs an optic nerve by reducing OBF while an IOP determination remains normal. A clinician having tonometer 10 to make determinations of both IOP and OBF can diagnose that deterioration of an optic nerve is caused by subnormal OBF, without any increase in IOP. The appropriate treatment based on the low OBF determination provided by tonometer 10 would then be aimed at improving OBF, rather than reducing IOP. Drugs now exist that improve blood flow generally, and these can be tried and the results monitored by rechecking the eye for both IOP and OBF. Drugs may also be developed that will aim especially at increasing OBF to the eyes while minimally affecting blood flow elsewhere.

Since OBF determinations have not previously been readily available to clinicians treating eyes for glaucoma, the effect of an OBF determination on eye treatment strategies has not been generally known. The ability of instrument 10 to provide such OBF information has many uses including monitoring an eye under treatment to be sure that the treated eye is not suffering from reduced OBF, which would call for a change in drugs being used in treatment. Having an OBF determination available from instrument 10 can also be beneficial in verifying that a drug aimed at increasing OBF in an eye experiencing normal pressure glaucoma has actually improved OBF. Knowing a tonography measure of the effectiveness of the eye's trabecular meshwork can also help monitor treatment determinations. For example, this can be used to determine the effectiveness of drugs intended to improve the working of the trabecular meshwork.

OPP and OBF, as measured by instrument 10, can be relevant to the health of other organs beside the eyes. Blood flow from the heart to the aorta and the carotid arteries proceeds upward above the neck, not only to the eyes, but also to the brain and other important organs. If blood flow in the carotid and other arteries serving the head is constricted, evidence of this may appear in an OPP or OBF measurement readily made available by instrument 10. More specifically, health ramifications of a sub-normal OPP measured in one or both eyes can imply potential problems such as transient ischemic attack (TIA) or cerebral vascular accident (CVA) stroke risks.

Previous tonometers have not been able to produce OPP and OBF signals, so their use has been limited to diagnosing eye problems. The fast and inexpensive availability of OPP and OBF measurements made possible by tonometer 10 allows OPP and OBF measurements to indicate possible cardiological problems for the head, generally. Ultrasonic examination of ocular blood flow in carotid arteries is available, but is a cumbersome and expensive test compared with the OPP and OBF information that is quickly gained from use of instrument 10. If instrument 10 detects sub-normal blood flow to one or both eyes, then an ultra sound evaluation and possible treatment might be able to forestall stroke risks such as TIA or CVA.

The OPP and OBF determinations available from instrument 10 also allow comparison of pulse pressure and flow to the two eyes of a single patient. An OPP or OBF that is subnormal for one eye and normal for another can indicate possible constriction of blood flow in an artery leading to the sub-normal eye. This can indicate that further testing such as an ultrasound examination of the appropriate carotid artery, is appropriate.

FIG. 5 illustrates the possibility of producing in a single examination excursion a signal that includes a preliminary value 27, a diastolic phase 26, and a systolic phase that includes ocular pulses 28. Such a signal would preferably extend for long enough to include two ocular pulses 28 so that microprocessor 50 can calculate a pulse rate. Of course, it is possible for such a signal to extend for a longer time, but this adds to the time that the prism is pressed against a patient's eye. A typical time needed to complete an examination excursion for diastolic purposes only is from 0.5 to 1.0 seconds. This would have to be extended somewhat to insure that two systolic pulses 28 occurred for pulse rate determination.

The signals shown in FIG. 5 can be used to determine an average or mean IOP 29 as previously described, along with ocular pulse signals 28. Although diastolic baseline 26 is sloped because of the signal being generated while prism pressing force increases over time, all the information necessary for determining IOP in the ways described above is available. An adjustment is needed for microprocessor 50 to take into account the sloping nature of the baseline signal 26 and the corresponding sloping nature of ocular pulse signals 28. Once the slopes are taken into account, all the necessary calculations for diastolic, systolic, and average or mean IOP can be calculated, along with OBF and a tonography measure.

The embodiment of FIG. 6 illustrates the possibility of producing a signal during increasing the prism pressing force as signal 26 slopes downward, followed by diminishing the prism pressing force as signal 26 slopes back upward. Such a reversing signal can also extend for long enough to include at least two ocular pulses 28 from which pulse rate can be determined. Again, after taking into account the slopes caused by prism force changes over time, a signal such as illustrated in FIG. 6 can be used to make all the determinations described above.

The embodiment of FIG. 7 schematically illustrates another way of determining a tonography measure by using tonometer 10. Beginning with preliminary signal value 27, microprocessor 50 is programmed to increase prism force in one of the previously described ways to produce signal 36 (from which any systolic pulses have been eliminated) to make a preliminary determination of diastolic IOP. Prism force is then increased to produce signal 37, and prism force is sustained at that level for a predetermined interval. The prism force level for producing signal 37 is sufficient to raise the IOP of the eye to a level above the IOP determination made during the excursion-producing signal 36. During the interval that prism force is elevated, as represented by signal 37, the eye that is subjected to the extra pressure attempts to accommodate to return its IOP to normal. The predetermined interval allowed for this is preferably in the range of a minute or two. Then prism pressing force is released, as represented by signal 38 and a new or subsequent IOP is determined as represented by signal 39 formed by increasing prism pressure from a preliminary value 27 b.

The prism force used to elevate the IOP to test the accommodating ability of the eye's trabecular meshwork is preferably sufficient to depress an ocular pulse signal during the predetermined interval that the prism force is applied. This results in the accommodation attempted by the trabecular meshwork to arise from the elevated IOP caused by the prism force as distinct from periodic accommodations following systolic pulses.

When elevated prism force is removed at the end of signal 37, the IOP of the previously pressurized eye reduces for a brief interval until the eye re-accommodates. During this interval a subsequent determination of diastolic IOP is made as represented by signal 39 showing that the IOP is reduced, and the eye has become temporarily softer. The difference between the higher IOP determined from signal 36 and the lower IOP determined from signal 39 gives a measure of the effectiveness of the trabecular meshwork of the eye. The healthier the eye, the greater will be the amount that the subsequent IOP is reduced from the preceding IOP.

This procedure is analogous to a known way of determining tonography by measuring IOP of an eye, and then while a patient is lying down, placing a weight on a patient's eye for an interval after which the IOP is again measured. Such a method is cumbersome and time consuming, since the patient has to lie down, and a weight has to be placed on the eye and then removed in between IOP determinations. By the method illustrated in FIG. 7, instrument 10 can make such a process much more efficient without requiring any weight and without requiring the patient to lie down. 

1. A method of measuring intraocular pressure (IOP) of an eye, the method using a tonometer having a programmed microprocessor controlling an actuator to press a prism with a variable and determinable force against a cornea of the eye while a source directs light to reflect from an applanation surface of the prism to a detector producing a detected light signal received by the microprocessor, which calculates IOP, the method being implemented by the microprocessor controlling the tonometer so that the method comprises: operating the tonometer to recognize a change in the detected light signal from a pre-contact value occurring before the prism contacts the eye to an initial contact value occurring upon initial contact between the cornea and the applanation surface, the initial contact value representing an initial applanation area that varies from one eye to another based on specific tear volume and biomechanical properties of the cornea of the eye being examined; operating the actuator to press the prism applanation surface against the cornea with increasing force applied during a time interval so that the detected light signal changes as an applanated area increases from the initial contact value by a predetermined amount; and determining IOP from a ratio of increased prism pressing force needed to increase the applanated area from the initial contact value by the predetermined amount.
 2. A tonometer operated by the method of claim
 1. 3. The method of claim 1 wherein the recognized change in the detected light signal upon initial contact is a reduction from the pre-contact value to the initial contact value.
 4. The method of claim 1 including mounting the prism on a counterbalanced arm so that the prism is free to move as the prism is moved into the initial contact.
 5. The method of claim 1 wherein the actuator prism pressing force is divided into predetermined increments of force applied at predetermined intervals of time.
 6. The method of claim 5 including using the time intervals as a measure of increased prism pressing force.
 7. The method of claim 1 including subtracting the initial contact value from the increased applanation area before determining IOP.
 8. A tonometer operated by the method of claim
 7. 9. A tonometer operated by the method of claim 1 including considering predetermined initial steps of actuator force as part of the initial contact value.
 10. A tonometer comprising: an applanation prism mounted on a rotationally counterbalanced arm; an electromagnetic actuator arranged to move the arm in response to a microprocessor to press the prism against a cornea of an eye being examined; the prism and the counterbalanced arm being arranged so that upon initial contact of an applanation surface of the prism with the cornea of the eye being examined, the arm is free to move, and the electromagnetic actuator does not press the prism against the cornea; the microprocessor being programmed to press the prism against the cornea after the initial contact is made to produce a predetermined departure from the initial contact; when the departure is a predetermined increase in applanation area, then the microprocessor is programmed to determine intraocular pressure from the prism pressing forces required to reach the predetermined increase in applanation area; and when the departure is a predetermined application of prism pressing force by the electromagnetic actuator, then the microprocessor is programmed to determine intraocular pressure from the difference between an applanation area of the initial contact and applanation areas occurring during the predetermined application of prism pressing force.
 11. The tonometer of claim 10 wherein the microprocessor is programmed to subtract an initial contact signal from signal values reached during the predetermined departures before determining intraocular pressure.
 12. The tonometer of claim 10 wherein the actuator presses the prism against the cornea in predetermined incremental increases in force applied at predetermined time intervals so that a total applied force and a total elapsed time are equivalent.
 13. The tonometer of claim 10 wherein the microprocessor, in determining the intraocular pressure, is programmed to consider predetermined initial steps of actuator force as part of the initial contact.
 14. A method of measuring intraocular pressure (IOP) of an eye by using an applanation tonometer having a prism with an applanation surface, a source directing light into the prism to be incident on the applanation surface, and a detector detecting light reflected from the applanation surface, the method comprising: mounting the prism on a rotationally counterbalanced arm that is free to move upon initial contact of the applanation surface with the cornea, surface tension of tears in the eye supplying a force to pull the freely movable prism against the eye to applanate an initial contact area whose size is a function of the tears and of biomechanical properties of the cornea of the eye; applying an electromagnetic force to the prism arm to press the prism against the eye after the initial contact to enlarge the applanated area; and determining IOP by using at least one of the following: a) the prism pressing force needed to enlarge the initial applanated area by a predetermined amount; and b) an increase in applanated area caused by a predetermined application of the electromagnetic force.
 15. A tonometer operated by the method of claim
 14. 16. The method of claim 14 wherein the IOP is determined by the electromagnetic force divided by the applanation area.
 17. The method of claim 14 wherein the electromagnetic force is applied in predetermined force increments at predetermined time intervals so that prism pressing force varies directly with elapsed time.
 18. A method of measuring intraocular pressure (IOP) of an eye using an applanation tonometer having a prism with an applanation surface mounted on a pivot arm, a source directing light into the prism to be incident on the applanation surface, and a detector detecting light reflected from the applanation surface, the method comprising: counterbalancing the pivot arm so that surface tension of tears is the only force pressing the applanation surface against the cornea as the applanation surface makes initial contact with the cornea; arranging the prism, the counterbalanced pivot arm, the source, and the detector so that light reflected from the applanation surface diminishes from a pre-contact value to an initial contact value representing an initial applanation area occurring upon initial contact of the prism applanation surface with the eye; applying electromagnetic force to the pivot arm to press the prism against the eye after the initial contact to enlarge the initial applanation area by a predetermined amount; and determining IOP by the ratio of the enlarged applanation area and the electromagnetic prism pressing force required to enlarge the applanated area to the predetermined amount.
 19. A tonometer operated by the method of claim
 18. 20. The method of claim 18 including applying the electromagnetic prism pressing force in predetermined force increments at predetermined time intervals to make the force increments and the time intervals equivalent.
 21. The method of claim 20 including determining the prism pressing force by the number of time intervals used in applying the prism pressing force.
 22. The method of claim 20 including subtracting the initial contact value from the enlarged applanated area.
 23. A method of measuring intraocular pressure (IOP) of an eye by using an applanation tonometer having a prism with an applanation surface, a source directing light into the prism to be incident on the applanation surface, and a detector detecting light reflected from the applanation surface, the method comprising: bringing the applanation surface of the prism into contact with the cornea of the eye so that surface tension of tears in the eye being examined pulls the applanation surface against the cornea, and the surface tension is resisted by corneal thickness and curvature of the eye being examined to produce an initial contact applanation area; pressing the prism against the cornea to increase a size of the initial contact applanation area by a predetermined amount; and determining IOP from the prism pressing force to increase the applanated area by the predetermined amount.
 24. A tonometer operated by the method of claim
 23. 25. The method of claim 23 including mounting the prism on a counterbalanced arm so that the prism is free to move as the prism is moved into the initial contact. 